It's a bit tricky to work with, but, I once made a converter with 3 NE592D. They are cheap, 90MHz bandwidth, made for video, differential in and differential out, except, they use a 'gain' resistor between 2 pins.
Add another 3 of them on the output if you want to mix positive video outs to the inverting inputs, but since you get an inverted output on the first 3, I just resistor mixed & fed the based of an 3x emitter follower 2N3904 amp driving the video cable with series 75ohm resistors on the output.
I tuned the system for 2v p-p before the 75ohm load.
Available in 14 pin, dip and smd, usually 25cents each.